1. Field of the Invention
The invention relates to a method of shearing metal strip and apparatus therefor. In particular the invention relates to the shearing of continuous strip, at the end of its production, into finite lengths which are coiled onto a spool.
2. Description of Related Art
In many strip production facilities the shearing has to take place whilst the strip is moving at high speed, and modern strip production requires the strip to be moving at higher and higher speeds to ever greater efficiencies in production costs. Typically in known systems, a rotary shear is used to cut metal strip which is moving. Such a conventional rotary shear is shown consists of two rotating drums each of which has a cutting blade. The two drums rotate in opposite directions, and are usually geared together. Separate guide means are usually provided to guide the strip onto a coil at the end of a rolling mill line and to transfer the strip to a second coil when a first coil is complete, whilst at the same time permitting the continuous production of the strip at an uninterrupted rate.
The strip passes between the two drums. When the strip is to be cut, the two drums are accelerated from rest until the peripheral speed of the blades is equal to the strip speed. The peripheral speed of the blades needs to be closely matched to the speed of the snip, otherwise the shear will cause a tension snatch in the strip which could damage the strip, the shear or other equipment in the line. After the blades have cut the strip, the drums are then decelerated back to rest.
With such a conventional rotary shear, since the drums cannot be allowed to rotate through the cutting position more than once, the maximum speed at which the strip can be cut is limited by the power required to accelerate and decelerate the drums within less than one revolution. To accelerate the drums to a higher speed requires more power, which in turn requires a bigger drive motor, which in turn increases the total inertia of the system and thus demands even more power.
It is possible for rotary shears of this type to have more than one set of blades. In these cases the problem of accelerating and decelerating the drums is even more pronounced because a smaller fraction of a revolution,is available for acceleration and deceleration.
Mannesmann AG, EP 0 771 603 A1 shows a drum with a moveable cutter blade. FIG. 1 shows a simple wedge-type actuator for position control, using hydraulic control. FIGS. 4 and 5 show a multi-step actuator, requiring less actuator movement and giving semi-stable full-in and full-out cutter positions. US Steel, U.S. Pat. No. 3,057,239 shows a cutter with a cylinder 2 having a fixed blade 30 and cylinder 4 a retractable blade 58, which is controlled (FIG. 3) by an axially movable element 46 having pins 52 which enrage in sloping slots 62 in the blade 58.